A comparative study of the fluorescence properties of vinyl-bridging anthracene and/or phenanthrene isomers

We studied and compared the photophysical and aggregation-induced emission properties of three isomers: trans-1,2-di(9-anthryl)ethylene (DAE), trans-1-(9-anthryl)-2-(9-phenanthryl)ethylene (APE), and trans-1,2-di(9-phenanthryl)ethylene (DPE). Both DAE and APE molecules have a twisted conformation due to intramolecular H···H steric hindrance. By contrast, this kind of steric hindrance drastically decreases in DPE molecule. From DAE, APE, to DPE, there are a blue shift in the optical spectra and a marked increase in the fluorescence efficiency. Mainly due to the restriction of intramolecular rotation, DAE displays an aggregation-induced emission behavior. However, the fluorescence efficiency of APE decreases from solution to solid state. Different from DAE and APE, DPE is a dual-state highly-efficient chromophore. The fluorescence efficiencies of its solution and the as-prepared powder (DPE-α) are 60% and 81%, respectively. J-aggregate was observed in the crystal structure of DPE-α, which correlates with its high emission efficiency. Interestingly, a new metastable polymorph of DPE was unexpectedly obtained by a solvent-antisolvent rapid precipitation. This new polymorph displays vapour- and temperature-induced crystal-to-crystal phase transition and fluorochromism.

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